1. Field of the Invention
The present invention relates to very fine cell polyether polyurethane foams and processes for producing the same.
2. Description of the Prior Art
Cellular polymers are prepared by generating a gas during the polymerization of a normally plastic or liquid reaction mixture. The gas causes foaming of the reaction mixture until a polymer sets or gels into a cellular structure known as a foam.
Polyurethane foams are prepared by reacting, in the presence of a blowing agent, polyisocyanates with active hydrogen containing substances such as hydroxyl containing polyesters and polyethers. Catalysts, such as organo-tin compounds and tertiary amines, and foam stabilizing surfactant/emulsifiers, such as silicone oils, are usually incorporated in the reaction mixture to control reaction rate, cell size and porosity. Due to the excellent properties of polyurethane foams, the usage of these foams for various applications has greatly increased over the last twenty to thirty years. For example, flexible polyurethane foam is used widely in comfort cushions (furniture, bedding, automotive); in textile (apparel, blankets); in industrial packaging and in insulation; as well as in household furnishings and the like. The versatility of polyurethane foam results in substantial part from the nature and variety of the starting materials as well as the manner in which the starting materials and the resultant foam are processed. Foams ranging widely in their properties--in density and stiffness, in tensile and tear, in resistance to compression set and fatigue, in flexibility and resilience, in durability and toughness and in hysteresis and char formation when exposed to flame--are obtained by selection and variation of the starting materials and processing conditions.
Many applications require polyurethane foam which is fine celled. For example, cosmetic applicators are made of fine celled polyester polyurethane foams which have a soft tactile feel. Some filter applications require fine cell foam to achieve a required pressure drop. It is known in the industry that the fatigue resistance of polyurethane carpet cushions is improved if the foam is made as fine celled as possible. Current state-of-the-art foams achieve a fine cell structure with about 87 pores (cell faces) per linear inch. A finer cell size is not obtainable using current mechanical techniques.
This invention achieves pore sizes in excess of 87 pores per linear inch by a chemical method. The conventionally used toluene diisocyanate is completely or partially replaced by an isocyanate based on diphenyl methane diisocyanate and polymeric versions thereof. This type of isocyanate also provides the advantage of producing higher load bearing foams with reduced amounts of graft polyols.